28 January 2018

CLOVER-2000 ARQ mode

This is a full transfer session recorded on 11135.0/USB 1414z between stations HQ4 and GANOB3 (supposedly belonging to an Egyptian network). Link is established and terminated using 2G-ALE 188-141A and data are sent using a CLOVER-2000 HF modem.

Quoting from wikipedia "CLOVER is the name of a series of modem waveforms specifically designed for use over HF radio systems: CLOVER-II was the first CLOVER waveform developed by Ray Petit, W7GHM, and HAL Communications in 1990-92. CLOVER-2000 (aka XCLOVER or 8 Tone CLOVER) is a higher-rate and wider bandwidth version of CLOVER developed in 1995. CLOVER-400 is a special 400 Hz wide waveform that was developed for Globe Wireless"

The eight tone pulses are sequential, with only one tone being present at any instant and each tone lasting 2 ms. Each frame consists of eight tone pulses lasting a total of 16 ms, so the base modulation rate of a CLOVER-2000 signal is always 62.5 symbols per second regardless of the type of modulation being used (Figure 2).

Fig. 2

In ARQ mode, all CLOVER Control Blocks use BPSK modulation and data (structured in 255-byte blocks) may be sent using five different types of modulation: BPSK, QPSK, PSK-8, 2APSK-8 (PSK-8 + 2-level Amplitude Shift Modulation), and 4APSK-16 (PSK-16 plus 4 ASM). The FEC broadcast mode of CLOVER-2000 is usually disabled although special formats are available for specific applications.Since CLOVER-2000 is an adaptive system, you may find different modulations in the same transfer, as confirmed by the analysis of the heard trasnmission: in the following Figures I analyzed the tone #4 in different data blocks:

Fig. 3 - PSK-2

Fig. 4 - PSK-4

Fig. 5 - PSK-8

Fig. 6 -2APSK-8

An example of 4APSK-16 modulation, from a different recording, is shown in Figure 7: you may note the 4 levels of amplitude modulation. The recording is from radioscanner.ru

Fig. 7

➤ updateThe sent message is encrypted BZIP2 compressed since the presence of the BZh11 sequence which marks the start of the compressed file (thanks to J.S4538). The name of the transferred file is in clear text "E_UNKNOWN_807_2701201.txt" in the header of the demodulated stream (thanks to KarapuZ for demodulation).

Fig. 8 - BZIP2 "BZh11" sequence

Fig.9 - file name

My friend J.S4538 also commented that the message is sent using the "2020 Binary Transfer Protocol" developed by HAL COMMUNICATIONS, the stations IDs in the play are ABORAMA1 (GANOB3 in ALE) and BASE41 (HQ4 in ALE).